EFFECT OF BAY K-8644 ON THE MAGNITUDE OF ISOFLURANE AND HALOTHANE CONTRACTURE OF SKELETAL-MUSCLE FROM PATIENTS SUSCEPTIBLE TO MALIGNANT HYPERTHERMIA

Isoflurane has a lesser ability than halothane to induce contracture i n malignant hyperthermia (MH) muscle in vitro. This does not necessari ly imply that isoflurane is not as potent an MH trigger as halothane i n vivo. A hypothesis was tested that in vitro treatment with Bay K 864 4, an activator of both the dihydropyridine receptors as well as the s odium channels of the T-tubules, potentiates isoflurane-induced MH-sus ceptible skeletal muscle contracture. In addition to the usual halotha ne-caffeine test, other muscle bundles were exposed to 10-mu-M Bay K 8 644-halothane and equipotent anesthetic concentrations (expressed in m ultiple minimum alveolar concentration [MAC]) of isoflurane either alo ne or combined with Bay K 8644. In 14 MH-susceptible muscle bundles, t he mean maximum contracture induced by 2 MAC isoflurane was 0.20 +/- 0 .22 g (mean +/- SD), and this value was significantly less than that o btained with 2 MAC halothane (0.68 +/- 0.40 g). Bay K 8644 did not ind uce muscle contracture on its own but consistently enhanced both the 0 .5 MAC isoflurane and halothane to the same maximal isometric tension (1.09 +/- 0.35 g and 1.11 +/- 0.37 g, respectively). Such an effect wa s not observed in the MH-nonsusceptible group. Under the conditions of this in vitro study, 0.5 MAC isoflurane appears to be as potent as ha lothane in inducing muscle contracture in skeletal muscle bundles from individuals susceptible to MH.